1. Field of the Invention
The present invention relates to an acoustic wave device for use as a bandpass filter, a resonator, or other suitable devices, and more particularly, to an acoustic wave device including a dielectric layer, such as SiO2, that is laminated on a piezoelectric substrate composed of LiNbO3.
2. Description of the Related Art
Heretofore, acoustic wave devices, such as surface acoustic wave devices and boundary acoustic wave devices, have been used for bandpass filters, for example, in communication equipment.
One example of a boundary acoustic wave device is disclosed in International Publication No. WO98/52279. FIG. 24 is a plan view and FIG. 25 is a partially enlarged front cross-sectional view that shows a relevant portion of a boundary acoustic wave device disclosed therein.
The boundary acoustic wave device 1001 includes a LiNbO3 substrate 1002. An IDT electrode 1003 is disposed on the substrate 1002. A polycrystalline silicon oxide film 1004 is laminated to cover the IDT electrode 1003. A polycrystalline silicon film 1005 is laminated over the polycrystalline silicon oxide film 1004.
A boundary acoustic wave excited at the IDT electrode 1003 is propagated such that the energy is concentrated in the polycrystalline silicon oxide film 1004 which is laminated between the LiNbO3 substrate 1002 and the polycrystalline silicon film 1005. Particularly, a polycrystalline silicon film/a polycrystalline silicon oxide film/a LiNbO3 are laminated in this order. That is, a boundary acoustic wave device having a three-medium structure is provided.
According to WO98/52279, since the polycrystalline silicon film 1005 is laminated over the polycrystalline silicon oxide film 1004, an acoustic wave excited at IDT electrode 1003 is reliably confined to the polycrystalline silicon oxide film.
International Publication No. WO2007/145057 describes a surface acoustic wave device that includes an IDT electrode provided on a LiNbO3 substrate and the IDT is covered by a laminated SiO2 film. In the surface acoustic wave device, the absolute value of the frequency-temperature coefficient TCF is reduced by forming the SiO2 film. In addition, by increasing thickness of SiO2 film in the range of about 0.24λ to about 0.35λ, where λ is a wave length, the occurrence of large spurious responses of a higher order mode is described.
In the boundary acoustic wave devices according to International Publication No. WO98/52279, boundary acoustic waves propagate through the polycrystalline silicon oxide film 1004 which is laminated between the LiNbO3 substrate 1002 and the polycrystalline silicon film 1005 by confining energy therein. There is a problem in that a large spurious response of the higher order mode occurs. It is known that the amount of the spurious response of the higher order mode can be suppressed by decreasing the thickness of the polycrystalline silicon oxide film. However, there is a problem in that the absolute value of the frequency-temperature coefficient TCF of the boundary acoustic wave device 1001 is increased by decreasing the thickness of the polycrystalline silicon oxide film. That is, there is a trade-off between suppressing the spurious response of the higher order mode and improving the frequency-temperature characteristic.
In the boundary acoustic wave device disclosed in WO98/52279, an acoustic velocity of a transverse wave through the polycrystalline silicon oxide film is less than acoustic velocities of the transverse wave through the LiNbO3 substrate and the polycrystalline silicon film. Since the polycrystalline silicon oxide film having a slow acoustic velocity is sandwiched between the polycrystalline silicon film and the LiNbO3 substrate, both having a fast acoustic velocity, a boundary acoustic wave excited at the IDT electrode can be reliably confined in the polycrystalline silicon oxide film. The boundary acoustic waves of a basic order mode and the higher order mode propagate through the structure.
The basic order mode means displacement is smaller in the out-going direction of the polycrystalline silicon oxide film, and one anti-node exists in the polycrystalline silicon oxide film, that is 0 order mode. The higher order mode means displacement is smaller in the out-going direction of the polycrystalline silicon oxide film, and one node exists in the polycrystalline silicon oxide film, therefore two anti-nodes in displacement directions opposite to each other exist, that is, in the 1 order mode.
The higher order mode having multiple nodes in the polycrystalline silicon oxide film can exist. However, since the response of such a mode is relatively small, other than the higher order mode described above, this is not problematic.
As described above, WO2007/145057 describes that a large spurious response of the higher order mode appears by increasing thickness of SiO2 film in the range about 0.24λ to about 0.35λ.
Accordingly, surface acoustic devices described in the International Publication No. WO2007/145057 also have a trade-off relationship between an improvement of the frequency-temperature characteristic by increasing the thickness of SiO2 film and a suppression of the spurious response of the higher order mode.